The invention relates to a method for controlling modulation of a lag compressor in a multiple compressor system, and more particularly to a method for controlling modulation of a lag compressor in a multiple compressor system wherein the pressure that the lag compressor is modulated around is determined by the actual lag compressor discharge pressure.
Conventional host controlled compressor systems are comprised of at least two compressors which together supply a compressed fluid at a required supply pressure, to meet the demand for use of the compressed fluid in the facility where the host controlled compressor system is located. Operation of the compressors is controlled by a microprocessor based host controller. Generally, the host controller receives signals from a supply line pressure sensor and compares the actual supply line pressure to a predetermined required supply line operating pressure range or pressure bandwidth, to determine whether it is necessary to modulate any of the system compressors.
In such conventional compressor systems, one or more of the compressors is designated the system lead compressor and one or more of the compressors is designated the lag compressor of the system. During operation of the conventional compressor system, the supply line pressure fluctuates primarily because of changes in the demand for the compressed fluid. As a result, it is necessary from time to time to modulate one or more of the system compressors in order to maintain the required system supply line pressure. The lag compressor is typically the system compressor that is modulated by cycling the lag compressor between fully loaded (100%) operation and unloaded (0%) operation.
The known method for modulating a compressor system is more fully explained by the following illustrative compressor system. FIG. 1 is a schematic representation of a conventional multiple compressor system 10 comprised generally of host controller 11, lead compressor 12, lag compressor 13, and supply line pressure sensor 14 which is flow connected to compressed fluid supply line 15. In the illustrative system 10, the compressed air demand requires that the lead compressor 12 be operated continuously, fully loaded, and that the lag compressor be modulated between unloaded and fully loaded operation.
During operation of system 10, the determination when to modulate the lag compressor is made by the host controller 11 and is based on the supply line pressure sensed by sensor 14. The host controller compares the actual sensed supply line pressure with the predetermined set point supply line pressure range stored in the host controller. When the supply line pressure is outside the acceptable supply line pressure range and is greater than the acceptable set point supply line pressure, the host controller sends a signal to lag compressor 13 and unloads the compressor until the actual supply line pressure is within the acceptable supply line pressure range. Lead compressor 12 remains fully loaded.
When the actual supply line pressure is outside the acceptable supply line pressure range and is less than the acceptable supply line pressure the host controller 11 sends a signal to the lag compressor 13 to fully load the lag compressor, and the lag compressor runs fully loaded until the actual supply line pressure is within the acceptable operating range. Then the host controller unloads the lag compressor. When the lag compressor is fully loaded, the lead compressor remains fully loaded.
This conventional method of modulating the lag compressor 13 by cycling between loaded and unloaded operation is repeated continuously during operation of the multiple compressor system 10.
There are a number of problems associated with the known method of modulating a lag compressor. First, modulation around the system supply line pressure may be inaccurate and is a complicated. System pressure drops may occur between the compressor discharge port and the supply line pressure sensor due to dryers or separator tanks which are flow connected to the supply line. If the lag compressor is modulated around the supply line pressure, the losses between the supply line sensor and lag compressor discharge must be considered in order to modulate the lag compressor effectively and accurately. The requirement that such losses be taken into account when modulating increases the complexity of the host controller logic and more data must be transferred between the host controller and the lag compressor. In a compressor system comprised of a large number of compressors, analysis and consideration of supply line losses can be quite burdensome.
Second, the benefits and efficiencies associated with state of the art precisely positionable inlet valves are not realized in a conventional, host controlled multiple compressor system that is modulated by cycling the lag compressors between fully loaded and unloaded operation.
Third, failure of mechanical and electrical components associated with loading and unloading of the compressor is accelerated as a result of the continuous, repetitive starting and stopping during compressor modulation.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.